Electric motor pump system and method

Abstract

An electric motor pump system includes a variable displacement pump (100), an electric motor connected to drive the variable displacement pump, a first control piston (120) configured to limit an output pressure characteristic of the pump, and a second control piston (160) controlled via a servo valve (150) according to an output speed of the electric motor. The second piston (160) is configured to maintain a substantially constant output flow of the pump as the output speed of the electric motor changes. The first and second control pistons (120, 160) may act on a yoke (130) of the variable displacement pump.

Description

[0001] This invention generally relates to pumps, including electric motor pumps (EMP) and systems, and methods of controlling electric motor pumps

[0002] US 4205590 A discloses a variable-displacement pump which is driven by an electric motor and which is controlled by an electrohydraulic servo valve. First and second control pistons can pivot a swashplate and thereby control the length of stroke of pump pistons.

[0003] KR 20120060441 A discloses a regulator for a hydraulic pump and EP 0576727 A1 discloses a control system for a variable displacement hydraulic pump with a two-lever type of feedback lever mechanism. In both these documents double-sided pistons are shown in which the piston at one end has a different diameter than the piston at the other enc However, these documents do not disclose a second control piston controlled via a servo valve according to an output speed of an electric motor determined based on an electric current provided to the electric motor from a variable frequency generator.

[0004] Some designs of electric motor pump systems include electronics, such as dedicated, extra, and / or high-power electronics, for controlling the pumps. Additional electronics may add weight and / or complexity, and may be difficult to service, particularly in situations where the pump system is installed in a relatively inaccessible location.

[0005] Accordingly, there is a desire to provide solutions or options that improve electric motor pumps and systems, for example by simplifying the systems and / or reducing their weight. Weight reduction is particularly important in aerospace applications and the like.

[0006] According to the invention, there is provided an electric motor pump system as it defined in claim 1. The electric motor pump system comprises a variable displacement pump; an electric motor connected to drive the variable displacement pump; a first control piston configured to limit an output pressure characteristic of the pump; a second control piston controlled via a servo valve. The first control piston has a larger diameter than the second control piston. The first control piston and the second control piston are arranged concentrically with respect to each other. In accordance with the invention the electric motor pump system further comprises a variable frequency generator configured to provide power to the electric motor, the variable frequency generator producing a constant voltage output with a variable frequency, wherein an output speed of the electric motor depends upon a frequency of the constant voltage output of the variable frequency generator. The second control piston is controlled via the servo valve according to an output speed of the electric motor, the second piston being configured to maintain a substantially constant output flow of the pump as the output speed of the electric motor changes. The first control piston and the second control piston act on a yoke of the variable displacement pump to vary the displacement of the variable displacement pump. However, the system of the invention can be used on any type of variable displacement pump, as long as speed compensation and pressure compensation can be carried out.

[0007] As the system is configured to limit the output pressure of the valve using a pressure-controlled piston, and to maintain a substantially constant output flow as the speed of the electric motor (which drives the variable displacement pump) changes by means of a servo valve, the system has less need for complex electronics, and so can be simplified and made lighter.

[0008] Preferred embodiments of the electric motor pump system are defined in the dependent claims.

[0009] In particular, means can be provided for sensing the pressure at the outlet of the pump. Any suitable means, such as a direct pressure sensor, can be used.

[0010] The first control piston can be controlled in any suitable manner, as long as it can respond to the outlet pressure of the pump. In a preferred form, the first control piston is controlled via a compensator spool.

[0011] The servo valve can operated in any suitable manner. However, it is preferred for the servo valve to be supplied with pressurized fluid from the outlet of the pump.

[0012] A preferred embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 is a schematic view generally illustrating a electric motor pump system according to a first aspect of the present invention; and Figure 2 is a schematic view generally illustrating an embodiment of a pump suitable for use in the pump system of Figure 1.

[0013] Figure 1 shows a schematic illustration of an electric motor pump system 10 for use in an aircraft. The system includes an aircraft engine 20, a variable frequency generator 30, an electric motor 40, and a variable displacement pump 50. The pump discharges an output fluid 60.

[0014] The aircraft engine 20 drives the variable frequency generator 30, which produces a constant voltage output with a variable frequency (which may be between about 380 Hz and about 800 Hz). The variable frequency generator 30 powers the electric motor 40, and may also control it. The electric motor 40 may be an induction motor, and the output speed of the motor may depend on the frequency of the output of the variable frequency generator. The motor 40 may be, for example, a line start permanent magnet electric motor.

[0015] The pump 50 is driven by the output shaft of the electric motor 40. As mentioned above, the pump 50 is a variable displacement pump, where the displacement (the amount of fluid delivered for each stroke) can be varied, for example by changing the angle of a yoke 130 (see Figure 2). The pump 50 may be speed compensated and pressure compensated.

[0016] The pump, and in particular the arrangements for speed compensation and pressure compensation, will be described in more detail with reference to Figure 2. In this Figure, the pump is generally denoted by the reference numeral 100.

[0017] With pressure compensation, the pump 100 is configured to modify the output flow based on the fluid pressure at the outlet of the pump (generally indicated as an "outlet circuit" and denoted by the reference numeral 110). For example, if the fluid pressure at the outlet 110 of the pump is high (and in particular, is above a threshold value), then the displacement of the pump can be reduced to reduce the output flow. The displacement can be controlled by means of a first control piston 160 that acts on a yoke 130 of the variable displacement pump 100. Movement of the first control piston 160 can be controlled by a compensator valve / spool 140, as shown in Figure 2, which is in communication with the outlet 110 of the pump.

[0018] With speed compensation, the pump 100 is configured to provide a substantially constant output flow across a range of speeds of the electric motor 40. To provide speed compensation, a servo valve 150 is controlled according to the speed of the output shaft of the electric motor 40. The speed of the electric motor 40 may be determined based on the electric current supplied to the electric motor 40 from the variable frequency generator 30, or may be measured directly using a resolver or a similar sensor connected to the motor output shaft (not shown). As the speed of the motor output shaft increases, the servo valve 150 controls a second control piston 120 to reduce the displacement of the pump 100 by changing the angle of the yoke 130, and thus maintain a substantially constant output flow. Likewise, if the speed decreases, the yoke 130 can be adjusted to increase the displacement to maintain a substantially constant output flow. The servo valve can be supplied with pressurized fluid from the outlet 110 of the pump 100.

[0019] The first and second control pistons 160, 120 can be disposed coaxially, and preferably concentrically. In a preferred arrangement, the first control piston is larger than the second control piston.

[0020] Thus, systems according to at least preferred embodiments of the present invention may be configured to provide pressure and speed compensated flow, such as across some or all frequencies of operation.

[0021] Power may be limited dynamically for running at any applied electrical frequency (that is, any frequency generated by the variable frequency generator 30), such as without control electronics, which may improve reliability and / or reduce system weight.

[0022] In some embodiments, the system may include a thermal sensor. At higher temperatures, power may be reduced.

[0023] Thus, the system does not necessarily need to include high power electronics to control the flow across various speeds of the motor (for example, it may not need to include a rectifier circuit and controller), and this can reduce the complexity and weight of the system, and / or improve the reliability of the system. Clearly, a reduction in weight and complexity and an improvement in reliability are highly desirable when the system is used in an aerospace application.

Claims

1. An electric motor pump system (10), comprising: a variable displacement pump (50, 100); an electric motor (40) connected to drive the variable displacement pump (50, 100); a first control piston (160) configured to limit an output pressure characteristic of the pump (50, 100); a second control piston (120) controlled via a servo valve (150); the first control piston (160) having a larger diameter than the second control piston (120); and the first control piston (160) and the second control piston (120) arranged concentrically with respect to each other, wherein the electric motor pump system (10) further comprises a variable frequency generator (30) configured to provide power to the electric motor (40), the variable frequency generator (30) producing a constant voltage output with a variable frequency, wherein an output speed of the electric motor (40) depends upon a frequency of the constant voltage output of the variable frequency generator (30); the second control piston (120) is controlled via the servo valve (150) according to an output speed of the electric motor (40) determined based on an electric current provided to the electric motor (40) from the variable frequency generator (30), the second piston (120) being configured to maintain a substantially constant output flow of the pump (50, 100) as the output speed of the electric motor (40) changes; and the first control piston (160) and the second control piston (120) act on a yoke (130) of the variable displacement pump (50, 100) to vary the displacement of the variable displacement pump.

2. An electric motor pump system as claimed in claim 1, wherein the system further comprises means for sensing the pressure at the outlet (110) of the pump (50, 110).

3. An electric motor pump system as claimed in claim 1 or claim 2, wherein the first control piston (160) is controlled via a compensator spool (140).

4. An electric motor system as claimed in any preceding claim, wherein the servo valve (150) is supplied with pressurized fluid from the outlet of the pump (50, 100).

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